The heat treatment of powder-metal (PM) parts presents a number of unique challenges, influenced primarily by material and density considerations. Common post-sintering processes include sinter hardening and conventional post-sintering heat-treatment operations. Let’s learn more.

Sinter Hardening

The process of sinter hardening is an alternative to conventional post-sintering hardening or case hardening and quenching, which takes place in a separate heat-treatment furnace. Sinter hardening is conducted inside the sintering furnace itself and involves rapid cooling after sintering to transform the microstructure to martensite. Cooling speeds vary by alloy, but rates in the range of 1-2°C/sec. (2-4°F/sec.) from 760-205°C (1400-400°F) are typical.[1] Carbon control of the furnace atmosphere is particularly important to achieving desired mechanical properties.

The chemistry and alloying method must be such that sufficient hardenability exists in the material to allow proper transformation of the microstructure for a given cooling rate. Typical sintering furnaces are equipped with water-jacketed cooling sections to cool parts using free convection. For some small parts, this cooling rate is sometimes (but not often) sufficient to achieve sinter hardening. For more massive parts, increased velocity and control of the rate of temperature loss are necessary to achieve the desired cooling rate (Fig. 1).

There continues to be considerable discussion among experts as to the importance of the flow direction of the furnace atmosphere. In many designs, atmosphere flows in a top-down manner, and the potential exists to develop a hardness gradient in the part, especially if it has significant mass. For this reason, all equipment may not perform the same, and some parts might be better suited for conventional heat treating.

Sinter hardening ferrous alloys has been widely used for a number of years for the production of press and sintered powder-metal parts with ultimate tensile strength (UTS) around 1,200 MPa and hardness values of 40 HRC at a sintered density of 7.2 g/cm3. The new generation of sinter-hardened materials raises these limits even further.

Advantages of sinter hardening include:

  • The need for a secondary quench-hardening treatment is eliminated.
  • The reduced distortion of parts due to the less-severe quench leads to better dimensional control.
  • Sinter-hardened parts do not need an oil-removal step prior to finishing operations such as plating.
  • The tempering of sinter-hardened parts in air is more straightforward than for quench-hardened parts.  
  • Parts that have been quenched in an oil bath retain a considerable amount of oil in their pores. If tempering at a temperature above 200°C (400°F) is required, the oil-quenched parts must first be tempered below 200°C (400°F) to burn off the entrapped oil prior to tempering at the higher temperature.

Conventional Heat Treatment

As stated earlier, the heat treatment of PM parts involves taking into account the density of the part being processed. Porosity (i.e., void space) means that PM parts are difficult to heat and cool.

Various consolidation and/or sintering techniques can be performed on a “green” part to achieve high density, including metal injection molding, hot isostatic pressing, P/M forging and liquid-phase sintering. However, conventional “press and sinter” is the most widely used method and results in a typical density range of 6.8-7.2 g/cm3. By comparison, wrought carbon steel (0.40%) has a density of 7.84 g/cm3.

In many cases, hardening and case hardening followed by quenching allow us to achieve the desired final part strength and hardness. Other popular processes include annealing, stress relief, tempering and steam treating.[1] Approximately 60% of current PM steels have their physical and mechanical properties enhanced after sintering by some form of secondary heat-treatment operations.

Ultimate tensile strength, hardness, wear, corrosion resistance and compressive strength can, in general, be improved by heat treating while properties such as impact resistance and ductility may be adversely affected. With this in mind, the selection of material chemistry and part process parameters are critical considerations for the successful application of heat-treatment techniques to PM parts.

The PM factor is a term used to describe a multitude of variables that influence the heat treatment of ferrous powder-metal parts. The most critical of these parameters include:

  • Part density
  • Material composition
  • Quenching or cooling method
  • Process factors
  • Equipment-induced variables

Factors such as the type of base iron or steel powder, as well as the amount and type of alloy additions and sintering parameters, are unique to the PM industry. When planning or executing a secondary heat-treatment operation, the most important variables to consider are density, microstructure, carbon and alloy content, process cycle and furnace atmosphere. In carburizing, for example, carbon is absorbed more rapidly than in conventional wrought materials (due to part density), and case depths develop much more rapidly (Fig. 2).

The quench media and the hardenability of the material have significant influence on as-quenched properties. Oil quenching, though less severe than water or brine, is preferred due to improved distortion control and minimized cracking. Control of oil temperature ensures load-to-load consistency, and the use of a “fast” (9-11 second) oil is preferred because of improved heat-transfer characteristics.

Since PM parts can absorb up to 3% oil (by weight), subsequent cleaning operations can be difficult. Incomplete cleaning leads to the creation of a great deal of “smoke” during tempering and potential safety concerns with respect to breathing these fumes. There is also a concern about fire due to the presence of large amounts of oil in the tempering furnace and/or ventilation ducts.

Quenching in water, brine or polymer as an alternative to oil can improve the rate of heat transfer. In many cases, however, this accelerates part corrosion due to residual fluid trapped near the surface. For this same reason, salt quenching can also create problems.

Temperature is one of the process variables that must be taken into consideration in secondary heat-treatment operations. In applications such as hardening and case hardening, temperature must be high enough to fully austenitize the material so that it will quench to a martensitic structure. Oil quenching, for example, may require a higher austenitizing temperature to achieve a structure similar to water or brine. It is also important to note that some secondary operations, such as tempering or steam treating, do not raise the parts to austenitizing temperature. However, the uniform distribution and dissipation of heat is a major factor in the consistency of the end product.

Time is another process variable that influences secondary heat treatment. Soak times up to 50% longer than wrought materials are typical. This is due to lower thermal conductivity of the porous P/M material.

Finally, the choice of atmosphere for hardening and sinter hardening of PM components is necessitated by the resultant metallurgy of the sintered material in combination with cost, productivity and the properties produced. Choices such as endothermic gas or nitrogen/methanol are popular for hardening and quenching, while hydrogen/nitrogen or dissociated ammonia/nitrogen atmospheres enriched with hydrocarbon gases are often used in sinter hardening.

Summary

Ultimately, the desired mechanical properties dictate the type of post-sintering heat treatment and atmosphere selection. With powder metallurgy increasing in popularity, heat treaters must be able to adapt their equipment to the needs of the PM industry.

 

References

  1. Herring, Daniel H., Atmosphere Heat Treatment, Volume I, BNP Media, 2014
  2. Herring, Daniel H., Atmosphere Heat Treatment, Volume II, BNP Media, 2015
  3. “Sintering,” Chapter 6, Höganäs PM School
  4. German, Randall M., Powder Metallurgy of Irons and Steels, John Wiley & Sons, Inc., 1998
  5. German, Randall M., Powder Metallurgy and Particulate Materials Processing, Metal Powder Industries Federation, Inc., 2005
  6. Pease III, Leander F. and William G. West, Fundamentals of Powder Metallurgy, Metal Powder Industries Federation, 2002
  7. Herring, Daniel H. and Jeremy C. St. Pierre, “Vacuum Carburizing of P/M Steels,” Industrial Heating, September 1987